Gaseous emissions from many thermochemical processes involve the emission of a number of pollutants ranging from heavy metal vapours (Hg, Cd, Pb and Zn), salt vapours (NaCl, KCl, KF, and NaF), acid gases (HF,HCl and SO.sub.2), organic compounds (dioxins, furans, hydrocarbons and solvents) as well as submicron particle emissions. The imposition of increasingly stringent environmental regulations to control these emissions has resulted in air pollution control devices becoming an integral component of thermochemical processes.
Typical attempts at reducing these pollutants include dry, wet and semi-dry scrubbing systems followed by particle removal in a baghouse or electrostatic precipitator. Wet scrubbing, by water spraying alone, is usually sufficient for control of HCl and HF emissions. For effective removal of SO.sub.2, sorbents are used. U.S. Pat. No. 4,704,972 discloses a method and apparatus for removing dust and water-soluble acid pollutants contained in smoke from the incineration of garbage wherein dust is removed from the smoke and the smoke is cooled by a heat exchanger below the dew point of gases therein whereby condensed water vapour traps acid pollutants for neutralization. The lowering of the temperature of saturated flue gas to below the dew point of acid pollutants does permit substantial condensation of said gases. However, low temperature emission control systems control systems suffer from a number of operational difficulties such as build up of insoluble salts on the walls of the vessel (scaling), corrosion and fouling of the vessel internals, difficulties in sedimentation of the produced salts, and clarification and recycling of vessel residue.
U.S. Pat. No. 4,281,518 (De-A-2902391) discloses the separation of certain components of a gas mixture therefrom by cooling the gas mixture by the addition of a liquid or cold inert gas to cause those components to go into a liquid or solid state. The condensed or precipitated components are collected and heated to form a liquid suitable for removal. This reference does not disclose contacting the gases with a powdered sorbent during cooling and adding an alkali metal to the sorbent to adjust the pH to about 9 to 11 for pH control.
It will be understood that the term "heavy metals" used in the specification and claims will include the metals, cadmium, lead, zinc, arsenic, antimony and selenium.